Tx ID as Asset ID (#926)

* Allow AssetLinks to be used in place of Assets in the Transaction Model and enforce `Transaction.transfer()` to only take an AssetLink

* Remove AssetLink's inheritance from Asset

* Remove id from the Asset model

* Fix get_txids_by_asset_id query for rethinkdb after removing asset's uuid

Because `CREATE` transactions don't have an asset that contains an id
anymore, one way to find all the transactions related to an asset is to
query the database twice: once for the `CREATE` transaction and another
for the `TRANSFER` transactions.

* Add TODO notice for vote test utils to be fixtures

* Update asset model documentation to reflect usage of transaction id

* Fix outdated asset description in transaction schema

docs/server/source/data-models/asset-model.md

@@ -5,18 +5,17 @@ To avoid redundant data in transactions, the digital asset model is different fo
 A digital asset's properties are defined in a `CREATE` transaction with the following model:
 ```json
 {
-    "id": "<uuid>",
     "data": "<json document>"
 }
 ```
 
-For `TRANSFER` transactions we only keep the asset id.
+For `TRANSFER` transactions we only keep the asset ID:
 ```json
 {
-    "id": "<uuid>"
+    "id": "<asset's CREATE transaction ID (sha3-256 hash)>"
 }
 ```
 
 
-- `id`: UUID version 4 (random) converted to a string of hex digits in standard form. Added server side.
+- `id`: The ID of the `CREATE` transaction that created the asset.
 - `data`: A user supplied JSON document with custom information about the asset. Defaults to null.